1. Field of the Invention
The present invention relates to a device for bending, i.e. varying the direction of, a guided optical light, in particular for use in telecommunication optical integrated circuit.
2. Description of the Related Art
In optical telecommunication systems, information is typically coded in short optical pulses by suitable optical sources, such as light-emitting diodes (LEDs) or semiconductor lasers, which pulses are transmitted along an optical-fibre network and received by photodetectors. Many different signals can be transmitted using a single wavelength of light by interweaving the pulses from different sources, a technique known as time-division multiplexing (TDM).
A simple way of increasing the amount of data that can be transmitted by a single optical fibre is to make the incoming electronic bits as short as possible. Current optical systems have achieved data rates up to 40 gigabits per second.
Recently, transmission capacity has been increased by dense wavelength division multiplexing (DWDM), which requires a very stable emitting laser and, at the receiver, very narrow linewidth filters and optical switches for separating individual wavelength channels and routing them to the appropriate destinations. Due to the large number of individual components in a DWDM system, integrated optical circuits have been developed. Integrated optical circuits may be either monolithic or hybrid and comprise active and passive components, typically realized on a semiconductor or dielectric substrate, used for coupling between optoelectronic devices and providing signal processing functions.
As described by R. L. Espinola et al. in “A study of high-index-contrast 90° waveguide bend structures”, OPTICS EXPRESS, Vol. 8, No. 9, pp. 517–528, 23 Apr. 2001, low-cost, highly-functional optical integrated circuits require a material system with relatively high refractive index contrast in order to increase the packing density of the optical elements, and also the use of sharp, e.g. 90°, bends. However, low-loss sharp bends cannot be easily achieved with standard waveguide technology because waveguide loss increases exponentially with the inverse bend radius.
New approaches to achieving sharp bends have therefore been considered.
For example, it has been proposed to use waveguide corner mirrors, which exploit strong modal confinement and total internal reflection (TIR) at the corner, as described by W. Yang and A. Gopinath in “Design of planar optical waveguide corners with turning mirrors”, Proceedings of Integrated Optics, Technical Digest Series, Vol. 6 (optical Society of America, Washington, D.C., 1996), pp. 58–63. With suitable mirror placement and angle, these structures can reflect the incident light with low-loss in the bend.
Another widely-studied approach to low-loss bends is that using high-index-contrast waveguides, whose strong light confinement properties allow performing complex waveguide interconnections within a small area. C. Manolatou et al, in “High-Density Integrated Optics”, Journal of Lightwave Technology, Vol. 17, No. 9, September 1999, pp. 1682–1692, show that by modifying the waveguide intersection regions into resonant structures with simmetry, right angle ends, T-junctions, and crossing with high transmission characteristics are also possible using conventional single-mode high index-contrast waveguides.
The Applicant observes that the very small dimensions of high-index-contrast waveguides cause difficulties in coupling light at their input and relatively high scattering losses.
Recently, the use of photonic crystal waveguides has been proposed for making high transmission 90°-bends, as described by A. Mekis et al. in “High transmission through sharp bends in photonic crystal waveguides”, Phys. Rev. Lett. 77, 3787–3790 (1996). In this case, photonic band-gap (PBG) materials are modified by inserting a line of defects that can support a localized mode having a frequency located within the photonic bandgap, as described by R. D. Meade et al. in “novel application of photonic band gap material: Low-loss bends and high Q cavities”, J. Appl. Phys. 75, 4753–4755 (1994). The defect line thus supports a local state and acts as a waveguide. An efficiency near to 100% has been observed at certain frequencies near the valence band edge for λ˜1.55 μm for a 60° photonic-crystal waveguide bend in E Chow et al., “Quantitative analysis of bending efficiency in photonic-crystal waveguide bends at λ=1.55 μm wavelengths”, Optics Letters, Vol. 26, No. 5, Mar. 1, 2001, pp. 286–288.
U.S. Pat. No. 5,526,449 discloses an optical circuit and a method for substantially eliminating radiation losses associated with optical integrated circuits and, in particular, bends in optical waveguides. The circuit and waveguide are fabricated on a substrate having a periodic dielectric structure. The periodic dielectric structure exhibits a range of frequencies of electromagnetic radiation that cannot propagate into the structure, i.e. a photonic band gap. Radiation at a frequency within the frequency band gap of the structure is confined within the circuit and waveguide by the periodic dielectric structure surrounding the circuit and waveguide.
U.S. Pat. No. 6,134,369 describes a compact optical waveguide employing a photonic band gap element as a reflector to enable a light beam to be reflected at angles greater than the critical angle. The photonic band gap element is a two-dimensional array of columnar holes formed in the substrate, the holes being filled with air or another material having a different dielectric constant than the substrate. The optical waveguide forms a right angle bend and first and second photonic band gap elements are formed on the inside and outside of the bend to deflect light which is incident on the waveguide at an angle greater than a critical angle defined by the materials that constitute the optical waveguide.
Besides development of photonic crystals devices having defects, behaviour of light in photonic crystals having regular periodicity, herein below referred to as “regular photonic crystals” for simplicity, has been investigated. For the purposes of the present invention, with “photonic crystals having regular periodicity” it is intended a photonic crystal wherein the characteristics of its periodic array do not vary at least in a region thereof of intended light propagation.
The article of P. Etchegoin and R. T. Phillips, “Photon focusing, internal diffraction, and surface states in periodic dielectric structures”, Physical Review B, Volume 53, Number 19, 15 May 1996-1, takes advantage of some analogies between electrons in semiconductors and electromagnetic waves in periodic dielectric structures for providing a method for calculation the band structure of a 2-D periodic dielectric structure. Moreover, this article deals with the phenomenon of photon focusing emitted by a source point in these structure, in analogy with the phenomenon of acoustic phonon focusing, showing what shape shall have the kx–ky diagram of the wave vector k to have focusing of light along predetermined directions.
The Applicant observes that the phenomenon of photon focusing, which has been studied only at theoretical level, would find only limited applications in integrated optics.
The article of Marko Lon{hacek over (c)}ar, Jelena Vu{hacek over (c)}kovi{hacek over (c)} and Axel Scherer, “Three-dimensional analysis of dispersion properties of planar photonic crystals”, Proceedings of PECS III conference (June 2001), St. Andrew's, Scotland shows that a planar (i.e. 2-D) photonic crystal may have, under certain conditions, a self-collimation effect in the second energy band (i.e. the energy band over the fundamental band). As disclosed in the article, these conditions determine a negative group velocity.
The Applicant observes that, for the time being, no practical applications have been shown of a beam of light (although collimated) having a negative group velocity.